1) Field of the Invention
The invention relates to a method of keeping a measuring device clean, the measuring device being arranged to measure at least one property of a moving web and comprising at least one measuring head with at least one measuring element.
The invention further relates to an arrangement for keeping a measuring device clean, the measuring device being arrangeable to measure at least one property of a moving web and comprising at least one measuring head with at least one measuring element.
2) Description of Related Art
In papermaking, characteristics of paper quality, such as thickness, are measured while a paper web to be manufactured is moving in a paper machine. Characteristics of paper quality are typically measured by means of traversing measuring devices, wherein measuring sensors are arranged in measuring carriages that traverse the paper web in the transverse direction thereof over the entire web width. However, measuring devices can also be stationary, in which case they constantly measure the same point in the transverse direction of the paper web.
Dirt and dust build up as the paper web is formed and dried, and when the web moves at a high speed in the machine, dirt and dust are spread around the paper machine. This dirt and dust sticks onto measuring elements or measuring windows of the measuring device, which causes distortions in the measurement result. Furthermore, in the case of high-speed paper machines and thin paper grades the dirt increases the number of holes the measuring device produces in the web. The problem of dirt has been further aggravated by the increased use of recycled pulp.
Cleanliness of measuring devices is very important in order to obtain an accurate measurement result. For example, the thickness of paper produced by a machine for producing newsprint is typically about 70 xcexcm, and the external accuracy of measurement to be achieved varies between 0.3 and 1 xcexcm. Dust and dirt gathering on the surface of a measuring element or a measuring window thus hinder the operation of a thickness gauge, in particular. Dirt also causes deviations in the measurement accuracy of profile, since even during one cycle of measurement which covers one back-and-forth movement over the paper web and which can last, for example, one minute, the thickness of the layer of dust and dirt gathering on the surface of the measuring element or window can change, at worst, dozens of micrometers.
The effect of dirt and dust on the measurement result has been compensated by standardizing the measuring devices at regular intervals. However, in view of an accurate measurement result this method is highly unreliable since new dirt accumulates and old dirt is removed continuously, and the amount of dirt can thus vary a great deal between rounds of standardization. The standardization trend exhibits great changes, yet there may have been even greater variations between instants of standardization than what the trend shows, such variations resulting from temporary accumulation of dirt.
Traversing measuring devices are currently cleaned during the standardization of the measuring device, i.e. when the measuring carriage and the devices therein are removed from the area of measurement outside the edge of the paper web. The measuring device in question and any other measuring devices in the same carriage are out of operation and no measurement results are thus obtained during cleaning. The measuring carriage is moved outside the web at desired intervals, typically for example once an hour, which is rather seldom regarding the accuracy of the measurement result. The measuring elements or windows can be cleaned for example manually by wiping them with a cleaning cloth or by blowing pressurized air onto the surface of the measuring element or window. Manual cleaning is laborious and requires service personnel. On the other hand, blowing pressurized air is not sufficient to remove accumulated dirt that is stuck onto the surface of the measuring element or window, such as a paper coating agent or resin provided in recycled pulp. The prior art also teaches blowing of pressurized air to keep a measuring window of a measuring device clean during measurement. An example of such an arrangement is disclosed in EP 0,974,828.
An objective of the present invention is to provide a new type of method and arrangement for keeping a measuring device clean.
The method according to the invention is characterized by cleaning the measuring element with cleaning agent during measurement.
The arrangement according to the invention is characterized in that the arrangement comprises means for feeding cleaning agent to clean the measuring element during measurement.
According to the basic idea of the invention, a measuring device arranged to measure at least one property of a moving web and comprising at least one measuring head with at least one measuring element, is kept clean by feeding cleaning agent to clean the surface of the measuring element during measurement. According to a preferred embodiment of the invention, the measuring element is kept clean by feeding cleaning agent to the surface of the measuring element or the moving web and by bringing the web and the measuring element into contact with each other, so that the moving web wipes the surface of the measuring element with the cleaning agent. According to another preferred embodiment of the invention, the measuring element is arranged to move up and down in the elevation of the measuring head, and the surface of the measuring element is brought into contact with the moving web by lifting or lowering the measuring element. According to a third preferred embodiment of the invention, the measuring element is fixed to the measuring head and the moving web is in continuous contact with the surface of the measuring element. According to a fourth preferred embodiment of the invention, cleaning agent is supplied to the surface of the moving web or the measuring element in the form of sprays with freely adjustable duration and interval. According to a fifth preferred embodiment of the invention, cleaning agent is continuously supplied to the surface of the moving web or the measuring element. According to a sixth preferred embodiment of the invention, cleaning agent is continuously supplied to the surface of the measuring element in order to prevent dust from sticking to the surface.
An advantage of the invention is that the cleaning of the measuring element and thus the measuring head does not require the measuring device to be moved outside the web, but the device can be cleaned and/or kept clean during the use thereof in measuring web properties. Therefore the measurement does not have to be interrupted, and high accuracy of measurement can be maintained even in dirty conditions. The use of the cleaning agent helps remove even persistent dirt from the measuring element, thus reducing the number of holes and other defects caused in the web by the measuring element. Depending on the speed at which the measuring element becomes soiled and on the conditions of use, cleaning agent can be fed either continuously or in the form of sprays with freely adjustable duration and intervals. The amount of cleaning agent needed to keep the measuring element clean is so small that it has no substantial effect on the web properties.
The arrangement according to the invention can be used similarly in connection with board manufacture, and in this specification the term xe2x80x98paperxe2x80x99 also refers to board and pulp.